UK light heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a match where the styles didn't complement each other, with constant grabbing marring the fight.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," Buatsi commented in the ring as he was met by boos from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the decision but the British boxer experienced the second defeat of his 28-fight career.
The Champion Claims Win in Slow Affair
Buatsi's progress has stalled in recent years through promotional disputes and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now is positioned trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who utilized a protective stance to edge the cleaner stanzas.
The spectators' acceptance diminished because of regular grabbing, with Parker going down several times during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific all-Manchester fight to win the European light heavyweight championship.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony rang in honor of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell disappeared, the quiet became a shout of "there's just one Hatton".
The action during the bout equaled the emotion. Arthur was knocked down by a left hook squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, heavy swelling forming under his orbital area as the fighters exchanged punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor edged the decision with scoring tallies of draw, narrow lead and clearer margin, but both fighters were determined and courageous, all the qualities their inspiration Hatton represented.
